drm/nouveau/dmem: remove set but not used variable 'drm'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/nouveau/nouveau_dmem.c: In function 'nouveau_dmem_free':
drivers/gpu/drm/nouveau/nouveau_dmem.c:103:22: warning:
 variable 'drm' set but not used [-Wunused-but-set-variable]
  struct nouveau_drm *drm;
                      ^
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NBen Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_dmem.c

@@ -100,12 +100,10 @@ static void
 nouveau_dmem_free(struct hmm_devmem *devmem, struct page *page)
 {
 	struct nouveau_dmem_chunk *chunk;
-	struct nouveau_drm *drm;
 	unsigned long idx;
 
 	chunk = (void *)hmm_devmem_page_get_drvdata(page);
 	idx = page_to_pfn(page) - chunk->pfn_first;
-	drm = chunk->drm;
 
 	/*
 	 * FIXME: